1. Field of the Invention
The present invention relates to a rechargeable battery charging method and a rechargeable battery charge controlling device that charge a rechargeable battery based on a target value of charging current, and a battery pack including this rechargeable battery charge controlling device.
2. Description of the Related Art
When charged, rechargeable batteries (hereinafter simply referred to as batteries) such as lithium ion batteries are typically charged at setting current and voltage values that fall within ranges not higher than the available maximum current and voltage (protection voltage for preventing the batteries from being overcharged) that are specified depending on the types of the batteries. Various charge controlling methods for this type of battery have been proposed in the past. In a most typical method, a battery is charged at charging voltage within a predetermined upper limit constantly at a setting current value in the beginning of charging operation, and is then charged constant at a setting voltage value after the charging current becomes lower than the setting current value (so-called constant-current and constant-voltage charging method, or constant-voltage and constant-current charging method). The level of the setting voltage value influences the life (deterioration degree) of a battery and the actual capacity of the battery as capacity that can be actually discharged from the battery. In terms of the level of the setting voltage value, the life and the actual capacity of a battery has a trade-off relationship. For example, the Japanese Patent Laid-Open Publication No. 2008-5644 discloses that the setting voltage value is gradually reduced with increasing the number of charging/discharging cycles of a battery whereby increasing the life of the battery but providing sufficient actual capacity in the case where the number of charging/discharging cycles is relatively small.
On the other hand, in the case where a battery pack with a plurality of serially-connected batteries is charged by the constant-current and constant-voltage charging method, it is necessary to take it into consideration that the battery voltages of the batteries will get unbalanced as the number of charging/discharging cycles increased. For this reason, the setting voltage and current values are often set at lower values in accordance with the highest battery voltage. For example, Japanese Patent Laid-Open Publication No. 2009-44946 discloses that, if the battery voltage of one of batteries exceeds a setting voltage value charging electric power is reduced so that the batteries are kept charged at the reduced setting voltage value and/or the reduced setting current value whereby suppressing increase of charging time and reduction of the actual capacity of a battery pack with the battery voltages of the batteries being unbalanced.
Batteries have a certain amount of internal resistance. When current flows at a setting current value through a battery in charging operation, voltage is produced by the internal resistance. In order to prevent abnormal rises of battery voltage, it is preferable that the setting current value be increased/reduced if the battery voltage when the battery is not charged/discharged is higher/lower. It is known that the internal resistance decreases/increases if the battery temperature gets higher/lower.
Japanese Patent Laid-Open Publication No. 2009-44946 discloses that the setting current values are optimized in accordance with the higher/lower level of battery voltage and battery temperature. On the basis of this method, in some cases, in order to reduce battery charging time, batteries are charged at a voltage within a range not higher than an upper limit that is higher than the protection voltage.